536 H. H. SWlNNEB'mN. 



and back to the level of the third branchial arch. From these 

 relationships it is evidently to be regarded as the representa- 

 tive of the basibranchial elements of the first three arches. 

 Behind it and between the lower ends of the fourth arch, 

 a small cartilaginous fourth basibranchial {hr. hA) has 

 appeared. 



The first representative of the pharyngo-branchial elements 

 is an oblong plate {hr. jj.^ — ^), but, owing to the absence of 

 any connections, it is impossible to decide definitely to which 

 arch it belongs. 



Huxley (58, p. 408) described the general appearance and 

 some of the main features of the first two arches of this stage 

 and of the adult, but both his figures and descriptions make 

 it evident that, owing to the methods at his disposal, several 

 interesting and important details escaped his notice. It will 

 therefore be better to give here a detailed description of all 

 the parts, even at the risk of some slight repetition. 



The liyoid arch is now quite complete. Mediauly there is 

 present a basihyal {hy. h.), broader in front than behind, and 

 much stronger than the copulare commune with which, 

 between the hypohyalia, it is connected only by pro-cartilage. 

 This element, therefore, chondrifies quite independently of 

 the copulare, and it thus differs from that of the salmon in 

 its mode of origin, as described by Stohr (p. 7), who speaks 

 of it as segmenting off from this. My observations, however, 

 agree with those of Pouchet (78, p. 57) on Gobius. 



The cartilage representing the combined ceratohyal and 

 epihyal (hy.) is still simple and bar-like ; but, proportionally 

 to the branchial arches, is much more massive than in the pre- 

 vious stage. Below, it fits into the concave outer surface of the 

 corresponding hypohyal ijiy. h.), which, as in the salmon 

 (Stohr, 83, p. 10), arises by differentiation of the pro- cartilage 

 surrounding its ventral ends. At its upper extremity it is 

 connected by a delicate pedicle of cartilage, the stylohyal, 

 with the remainder of the arch. This little element arises as 

 a separate chondrification in the pro-cartilaginous tract 

 described in this region for Stage I. 



